An electret condenser microphone (hereinafter referred to as “ECM”) is widely used in a mobile telephone, a digital recorder such as an IC recorder, a personal computer, etc. The ECM has a low output frequency and extremely high output impedance. Accordingly, a signal to be inputted to an amplification circuit connected to the ECM is biased by a high resistance of a resistor.
A bias resistor of high resistance is formed in a semiconductor device so as to match manufacturing steps of a MOSFET to be formed in the semiconductor device. For example, a resistor circuit using a drain-source resistance of a MOSFET is often used as the bias resistor of high resistance. In this case, the gate voltage of the MOSFET is a voltage in a range lower than the threshold voltage.
The bias voltage generated by the resistor circuit changes, when a signal having a large signal amplitude is inputted into the bias resistor circuit which is connected to an input end of an amplification circuit. This change of the bias voltage causes change of the bias resistance value, which deteriorates the distortion characteristic of the amplification circuit and may increase a total harmonic distortion.